Electrostatic discharge (ESD) is the sudden flow of electricity between two objects caused by contact, an electrical short, or dielectric breakdown. ESD can be caused by a buildup of static electricity by tribocharging, or by electrostatic induction. ESD includes spectacular electric sparks, but also less dramatic forms which may be neither seen nor heard, yet still be large enough to cause damage to sensitive electronic devices. ESD can cause a range of harmful effects, as well as failure of integrated circuits (ICs).
Electrostatic discharge (“ESD”) protection circuits are needed for ICs. The ESD protection circuits provide a path to bypass current from the terminal to a ground or from the terminal to a power supply rail, so that the current due to an ESD event bypasses the internal circuitry. Voltages far in excess of the normal operating voltages, in both positive and negative magnitudes, are observed during short duration electrostatic discharge events. The ESD protection circuits prevent the corresponding ESD current from destroying sensitive components in the protected IC.
An ESD protection circuit can be triggered in response to a trigger voltage over a threshold, and then safely conduct ESD stress current through an alternative path and thus protect the internal circuitry. After the ESD protection circuit is triggered, it will remain active as long as a voltage over a “holding voltage” is present.
Snapback devices such as silicon controlled rectifier (SCR) circuits and bipolar junction transistors (BJTs) may be used as ESD protection circuits. However, SCR circuits may have potential burn out risks caused by mistakenly triggered ESD events since the trigger voltage of a SCR circuit may change when ESD events generate waveforms with different rising times. On the other hand, the PNP BJTs may have a larger area requirement which may prevent them from being used in many cases, even though they may be safer and more suitable for more applications. Improved ESD protection circuits are needed.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the illustrative example embodiments and are not necessarily drawn to scale.